1. Field of the Invention
The present invention generally relates to transmission devices connected to an optical transmission line, and more particularly to a communication device which has the function of amplifying a signal light and communicates with a remote communication device in two ways.
Recently, communications utilizing an optical fiber as a transmission medium have been practically used, and a long-distance signal transfer has been realized. In order to advance such optical communications, it is desired to further improve the light amplifying technique and the two-way communication technique.
2. Description of the Related Art
Nowadays, a transmission device used in an optical communication system is equipped with a light amplifying device which amplifies a light to be transmitted to an optical fiber in order to enable a long-distance light transfer.
FIG. 1 is a block diagram of a conventional light amplifier device 11, which is made up of an exciting light source 12, a wave combiner 13, an optical amplifier 14 of an optical fiber type, and an optical isolator 15. The combiner 13 combines a light to be transmitted and an exciting light from the source 12 together. The optical amplifier 14 optically amplifies the output signal of the combiner 13. The amplified signal light passes through the optical isolator 15, and is output to an optical fiber cable 16.
The optical amplifier 14 is formed of, for example, a rare-earth doped optical fiber, such as an erbium-doped optical fiber. The optical isolator 15 isolates a reflected light from the transmission line in order to prevent the reflected light from being applied to the optical amplifier 14. If the reflected light is applied to the optical amplifier 14, the optical amplifier 14 will oscillate in an unstable state.
The light amplifier device 11 shown in FIG. 1 cannot realize the two-way optical transmission because the optical isolator 15 is connected to the optical fiber cable 16.
A two-way transmission device which has the light amplifying function is disclosed in Japanese Unexamined Publication No. 58-115948 or No. 6-53585. The two-way transmission devices disclosed in the above publications do not use an optical isolator as described above, and thus the light amplifiers equipped therewith may oscillate in the unstable state.
It is possible to modify the structure shown in FIG. 1 in order to enable the two-way optical transmission by positioning the combiner 13 between the optical isolator 15 and the optical fiber cable 16. In this case, a received signal light from the cable 16 bypasses the optical isolator 15 and is applied to a receive part (not shown). However, the above modification has a problem caused when the cable 16 is broken. The amplified light transmitted to the cable 16 from the optical amplifier 14 is reflected by an end surface of the broken part of the cable 16. The reflected light is applied to the receive part, which cannot discriminate whether the received light is a signal light from a remote transmission device or the above reflected light. In other words, the above modification does not have any function of determining whether the received light is a reflected light caused by a break of the cable 16. Further, the reflected light emitted from the end surface of the broken part of the cable 16 has a high energy level, which may damage a maintenance person.